Today, thin client applications often rely on the infrastructure of the WWW to deliver their user interfaces (UIs) to clients. While this approach does not require the deployment of application logic on the client, Web-based UIs typically do not provide the same level of usability as window-based UIs. We therefore present a UI framework that combines the flexibility of a thin presentation logic with the usability of a full-featured UI: Our approach uses an XMPP-based instant messaging infrastructure to exchange XUL interface descriptions and events between the application logic on the server and a generic UI rendering engine on the client